Getting index of duplicate items in a list in c#

Question

I am looking for a way to get the index of all elements in a list from a keyword search within the list. So for example my list has:

Hello World
Programming Rocks
Hello
Hello World
I love C#
Hello

Now from this list of strings, i want to get all the indices of elements that say Hello World. I have tried the following but it only returns the first index it finds that has my search criteria:

    for (int i = 0; i< searchInList.Count; i++)
        foundHelloWorld[i] = searchInList.IndexOf("Hello World");

Anyone know of a way to do this?

Thanks

Answer 1

searchInList.Select((value, index) => new {value, index})
    .Where(a => string.Equals(a.value, "Hello World"))
    .Select(a => a.index)

If you're trying to search for more than just "Hello World", you could do

searchInList.Select((value, index) => new {value, index})
    .Where(a => stringsToSearchFor.Any(s => string.Equals(a.value, s)))
    .Select(a => a.index)

Answer 2

Since you know you're looking for ALL occurrences and therefore you must traverse the entire list anyway, you'll gain a lot of readability over using IndexOf by simply examining each element yourself:

var i=0;
foreach(var value in searchInList)
{
   if(value == "Hello World")
      foundHelloWorld.Add(i); //foundHelloWorld must be an IList

   i++;
}

You can also use an overload of the Linq Select method that incorporate's the element's index in the source collection; this should be highly readable (and thus maintainable) to Linq-experienced programmers:

foundHelloWorld = searchInList
                     .Select((v,i)=>new {Index = i, Value = v})
                     .Where(x=>x.Value == "Hello World")
                     .Select(x=>x.Index)
                     .ToList();

The above code takes the list and transforms the string into a simple anonymous type incorporating each item's place in the original list. Then, it filters down to only matching elements, and then it projects out the Index (which didn't change through the filtering) into a new List object. However, all this transformation will make this solution perform slower, because this statement will traverse the entire list multiple times.

Answer 3

A little ugly but will work:

    var searchInList = new List<string>();

//Populate your list

    string stringToLookUp= "Hello world";
    var foundHelloWorldIndexes = new List<int>();

    for (int i = 0; i < searchInList.Count; i++)
        if (searchInList[i].Equals(stringToLookUp))
            foundHelloWorldIndexes.Add(i);